Two factors at odds: "have experienced crew on the station, efficient since day zero" vs "have more astronauts with the station operations experience, not depending on the same small crew" - how often do astronauts return to ISS after having "served a term" there? What's the average ratio of astronauts returning to ISS vs newcomers, first time there?

$\begingroup$The impact of being a newcomer is reduced greatly by years of training.$\endgroup$
– HobbesDec 28 '16 at 9:25

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$\begingroup$@Hobbes: I recall - I think that was Scott Kelly's AMA - where he talked astronauts have a difficult time adapting to zero-G over the first few days/weeks - using too much force moving around, bumping into everything, suffering from nausea, needing the vomit bag with them during the first days - it takes a considerable time of adaptation before they can get to full productivity.$\endgroup$
– SF.Dec 28 '16 at 14:33

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According to Wikipedia, there have been 226 visitors to the ISS. Of these, 1 person made five visits, 6 people made four visits, 28 made three, and 84 made two. This means that 119 of the visitors made more than one visit, which is ~53% of all visitors. However, this is just people going to the ISS. Some of these missions were Space Shuttle missions, where the crew of the Space Shuttle was only there for a short amount of time before returning to Earth.

For crew members only, I went through this list to count the number of times each name showed up. I found that, of the 106 crew members, 2 went four times, 5 went three times, and 21 went two times. This means that only 26% of ISS crew members had previously been on the ISS.

If you examine the aformentioned list, it appears that since 2009, when we stopped using the Space Shuttle to ferry crew members, the common practice is for the ISS to have a crew of 6 members. Whenever a new Soyuz capsule docks, the three members who had been there longest depart, leaving the ISS with three veterans and three new crew members. Then, when the next Soyuz arrives, the old veterans leave, once again leaving three veterans and three new members.

$\begingroup$@Digger I guess I was a little unclear there; the last STS mission to the ISS was in 2009.$\endgroup$
– PhiterosJan 28 '17 at 3:58

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$\begingroup$The final STS mission was STS-135, which flew, in 2011, to the ISS.$\endgroup$
– DiggerJan 29 '17 at 22:39

$\begingroup$@Digger Once again, my mistake. According to the Wikipedia list, STS-128 was the last to carry crew members to the station. Other missions went, but they only carried equipment and supplies.$\endgroup$
– PhiterosJan 29 '17 at 23:34